Surface measurements of direct and diffuse voltages at UV wavelengths were
made at the T1 site during the MILAGRO (Megacity Initiative: Local and
Global Research Observations) field campaign in March 2006, using a
multifilter rotating shadowband radiometer (UV-MFRSR). We used the MFRSR
data, together with measurements from a co-located CIMEL Sun photometer at
the site operating as part of the AERONET network, to deduce aerosol single
scattering albedo (ω) at 368 and 332 nm for four cloud-free days during the
study. Our retrievals suggest that T1 aerosols with aerosol extinction
optical depth &tau;<sub>368</sub>&gt;0.1 that are influenced by Mexico City emissions,
blowing dust, and biomass burning, are characterized by low &omega;<sub>368</sub>=0.73–0.85
and &omega;<sub>332</sub>=0.70–0.86, with small or no spectral variation of
ω between 368 and 332 nm. Our findings are consistent with other published
estimates of ω for Mexico City aerosols, including those that suggest that
the absorption attributable to these aerosols is enhanced at UV wavelengths
relative to visible wavelengths. We also demonstrate, via sensitivity tests,
the importance of accurate τ and surface albedo measurements in ω retrievals
at UV wavelengths.